The present invention relates to a structure for applying a pre-load to a rolling bearing and a three-dimensional survey device having the structure.
A publicly known three-dimensional survey device may include a 3D laser scanner that may be used mounted on a tripod. Such a 3D laser scanner scans an object to be measured by emitting distance measuring light thereto, to measure a distance to a measured point, and thereby obtain three-dimensional data. The object to be measured is three-dimensionally scanned with the distance measuring light that is emitted from a scanning mirror while the scanning mirror rotates in a horizontal plane and in a vertical plane. For example, a commonly used three-dimensional survey device may have a scanning mirror that is supported rotatably in a vertical plane. This three-dimensional survey device may scan the entirety of an object to be measured, with distance measuring light that is emitted from the scanning mirror, while the entirety of the device rotates in a horizontal plane. One such device is disclosed in Japanese Patent No. 6120521, for example.
The three-dimensional survey device disclosed in Japanese Patent No. 6120521 includes a scanning unit that rotates a scanning mirror in a vertical plane. FIG. 10 shows an example of an existing scanning unit of this kind. With reference to FIG. 10, the reference sign 500 indicates a housing, the reference sign 600 indicates a vertically rotatable shaft that is rotatably supported by the housing 500, and the vertically rotatable shaft 600 protrudes from the housing 500 in a downward direction in the drawing and has a scanning mirror 700 that is fixed at a lower end part. The scanning mirror 700 rotates with the vertically rotatable shaft 600. The vertically rotatable shaft 600 is horizontally provided to a three-dimensional survey device, and thus, the scanning mirror 700 rotates in the vertical plane. The housing 500 contains a motor 800 that rotatably drives the vertically rotatable shaft 600.
The housing 500 includes a cylindrical housing body 510 and a disk-shaped lid member 520A. The housing body 510 contains the motor 800. The lid member 520A closes an opening at the upper side of the housing body 510 by being fixed to a rim of the opening of the housing body 510 with a screw 590. The vertically rotatable shaft 600 is rotatably supported by bearings 560 and 570 that are paired rolling bearings respectively fitted to a bottom of the housing body 510 and the lid member 520A. Each of the bearings 560 and 570 has an inner space that can cause slight backlash, and therefore, the bearings 560 and 570 can cause decrease in measurement accuracy if no countermeasures are taken. In view of this, a pre-load is applied to the bearing 570 in an upper side by downwardly pressing an outer ring 570a with the lid member 520A, to reduce generation of backlash. This structure is described in detail below.
As illustrated in FIG. 11, the lid member 520A includes a cylindrical sleeve part 521 and a disk part 525. The sleeve part 521 engages with the outer ring 570a of the bearing 570. The disk part 525 has a predetermined thickness that imparts sufficient stiffness and is concentric with the sleeve part 521. The disk part 525 has multiple insertion holes 525a that are formed at a periphery for inserting screws 590. On the other hand, the housing body 510 has screw holes 510a that are formed at an end surface of the rim of the opening in correspondence with the insertion holes 525a. To fix the lid member 520A to the housing body 510, the sleeve part 521 is put on the outer ring 570a of the bearing 570 so that an engaging part 521a will engage with an upper end surface of the outer ring 570a via a waved washer 595, and the screws 590 are passed through the respective insertion holes 525a and are screwed and fastened to the screw holes 510a. This fixing structure makes the waved washer 595 elastically press down the outer ring 570a, thereby applying a pre-load to the outer ring 570a. The waved washer 595 is an elastic member for applying a pre-load, and instead of the waved washer 595, the elastic member may be a spring or other part.
The pre-load applied to the bearing 570 is controlled to be in a predetermined range while a necessary bearing performance is obtained, and the pre-load is controlled by adjusting a load that generates the pre-load. Using the pre-load applying structure illustrated in FIG. 11, it is relatively easy to adjust the pre-load because the pre-load can be varied by changing the thickness or the elastic deformation amount of the waved washer 595. However, the elastic member such as the waved washer 595 may rotate and contact the outer ring 570 at a part different from an expected part, which changes the applied condition of the pre-load and causes change in accuracy of applying the pre-load.
In view of this, a pre-load applying structure as illustrated in FIGS. 12 to 14 that overcomes the above drawback has been developed. Note that structural components in FIGS. 12 to 14 that are the same as those in FIGS. 10 and 11 are represented by the same reference signs. The structure illustrated in FIGS. 12 to 14 does not have an elastic member such as the waved washer 595 for applying the pre-load but does have the lid member 520A with its shape changed. As illustrated in FIGS. 13A to 14, the new lid member 520B has the cylindrical sleeve part 521 in a similar manner as the lid member 520A, but, instead of the disk part 525, the lid member 520B has an outer peripheral annular part 522 and a plate spring part 523. The outer peripheral annular part 522 has a predetermined thickness that imparts sufficient stiffness and is concentric with the sleeve part 521. The plate spring part 523 is formed so that the thickness is thin between the sleeve part 521 and the outer peripheral annular part 522. Upper surfaces of the outer peripheral annular part 522 and the plate spring part 523 are flush with each other, and the sleeve part 521 upwardly protrudes from these upper surfaces. The outer peripheral annular part 522 has a downwardly projecting annular protrusion 522b that is formed at an inner peripheral edge. The lid member 520B has a lower surface to which a recess 520c is formed at an inner side of the protrusion 522b. The lid member 520B is an integrally formed member that is formed by cut machining a material such as aluminum.
As illustrated in FIG. 14, to fix the lid member 520B to the housing body 510, the sleeve part 521 is put on the outer ring 570a of the bearing 570 so that the engaging part 521a will engage with the upper end surface of the outer ring 570a, the protrusion 522b is fitted to the inside of the rim of the opening of the housing body 510, and the screws 590 are passed through the respective insertion holes 525a and are screwed and fastened to the screw holes 510a. In the condition in which the engaging part 521a of the sleeve part 521 engages with the upper end surface of the outer ring 570a, the outer peripheral annular part 522 of the lid member 520B and the end surface of the rim of the opening of the housing body 510 are made to have a clearance therebetween, into which a washer 580 for adjusting a pre-load is interposed as a spacer. With this structure, the fastened screw 590 presses the outer peripheral annular part 522 of the lid member 520B and thereby bends and elastically deforms the plate spring part 523, and the washer 580 is pressed by the outer peripheral annular part 522 and tightly fits to the rim of the opening. Thus, the fastening is completed. The elasticity of the plate spring part 523 in the elastically deformed condition makes the engaging part 521a of the sleeve part 521 press down the outer ring 570a, and this force is a load serving as a pre-load to the outer ring 570a. 
To adjust the pre-load to be generated in the structure illustrated in FIG. 14, the clearance between the outer peripheral annular part 522 of the lid member 520B and the end surface of the rim of the opening of the housing body 510 is measured before the screw 590 is fastened, a washer 580 having a thickness corresponding to the clearance is interposed into the clearance, and then, the lid member 520B is fixed. That is, multiple washers having different thicknesses are prepared, and a washer having a thickness corresponding to the clearance is selected therefrom, whereby the pre-load is controlled. This structure is not prone to generate imbalance of the pre-load that is applied to the outer ring 570a because the engaging part 521a contacts the entire circumference of the upper end surface of the outer ring 570a, and this structure can eliminate the drawback in the structure illustrated in FIG. 11.
However, the plate spring part 523 of the lid member 520B exhibits small elastic deformation and has a small adjusting range for the pre-load because the entirety of the lid member 520B is an integrally formed member while the plate spring part 523 is held between the sleeve part 521 at the inner peripheral side and the outer peripheral annular part 522 at the outer peripheral side. This makes it necessary to prepare a large number of washers having different thicknesses and forces a complicated pre-load control. Further, since the thickness of the plate spring part 523 is difficult to make uniform with high accuracy, the load should be adjusted corresponding to the thickness of the plate spring part 523, which also makes the control complicated. Additionally, the complicated shape of the lid member 520B itself increases cost.